There is an unmet need for an inner ear imaging technique that can provide anatomic and physiologic information in live subjects in a relatively nondestructive manner. Such a tool might shed light on fundamental issues concerning hearing function and loss, enable clinically significant diagnostics, and guide future surgical efforts to restore hearing function by micromanipulation. The results of the current study suggest that the use of FME deserves exploration for such applications. This view is supported by a number of previous reports that suggest the clinical potential for cochlear microendoscopy using imaging of scattered light (35
In one report, flexible fiberoptic endoscopes (0.35–0.89 mm) were used to evaluate cochleae of patients undergoing repeat cochlear implantation (36
). The authors reported that endoscopes were useful in identifying anatomic obstructions of the cochlear scalae that were not always apparent during preoperative imaging studies (36
). In another study, endoscopes were used to guide laser recanalization of a cadaveric model of ossified cochlea (35
). However, in none of these previous endoscopy applications have the images produced by scattered light been of sufficient contrast and resolution to allow visualization of cochlear microcirculation. Thus, to date, nearly all studies of cochlear blood flow have involved measurements of bulk flow speeds using laser Doppler techniques.
Laser Doppler flowmetry allows determination of blood flow speeds based on the Doppler frequency shift of a reflected laser beam. Placement of the Doppler probe outside the bony otic capsule allows preservation of the cochlea. However, with this experimental configuration, bone blood flow within superficial vessels of the capsule also contributes to the Doppler signal (22
). Thus, there is the potential for inaccuracy, especially because the otic capsule and its surrounding mucosal vasculature are supplied by arteries other than the cochlea (5
). Moreover, vessels closest to the probe contribute a disproportionately greater signal than more distant vasculature, so the small blood supply to the organ of Corti and to other sensitive neural elements might be obscured by the larger and much closer vessels in the lateral cochlear wall (5
). Through comparisons with measurements of cochlear blood flow using microsphere-based techniques, the contribution to the laser Doppler signal from blood flow in the otic capsule has been estimated to be 30 to 40% of the total Doppler signal in rats (24
). This contribution from bone flood flow is expected to be higher in the thicker walled human cochlea (5
). The optical transmission coefficient through the human promontory bone was estimated in one study to be only approximately 1 to 2% for red light of 632-nm wavelength, as compared with values measured in rats and guinea pigs of approximately 15% and approximately 6%, respectively (50
). Another study found considerably higher transmission coefficients through the human promontory (49
). Nonetheless, Doppler flowmetry has been applied clinically (15
). However, the inability of Doppler methods to provide data on flow speeds within specific areas of the cochlea is a major limitation that intravital optical imaging is well suited to address.
Intravital fluorescence microscopy is an optical imaging method that can provide detailed information on blood flow within single cochlear vessels. The technique relies on a long-WD microscope objective to view red blood cells in the cochlea and has been used to determine blood flow speeds within cochlear regions that are visible through a window in the otic capsule (2
). However, sacrifice of surrounding structures has been required for placement of the microscope objective sufficiently close to the cochlea to obtain images. Thus, intravital microscopy appears to be best suited for use in animal subjects with easily accessible cochleae, such as the guinea pig, and to our knowledge has not been applied in a clinical setting. However, an optical imaging approach that affords resolution similar to that of microscopy but that does not require sacrificing surrounding structures might hold significant potential for both clinical and research applications. FME appears to be just such an approach. Moreover, we have shown that by inserting an endoscope probe within the cochlea, rather than opening a window in the otic capsule as is done in intravital microscopy studies (32
), FME can provide direct views of blood flow in cochlear areas such as the round window and basilar membrane that are not visible through an otic capsule window.
Such use of the endoscope probe within the cochlea has allowed us to estimate red blood cell speeds within the basilar membrane. Unlike Doppler flowmetry, microendoscopy offers the ability to image cochlear microanatomy and blood flow concurrently and does not confound cochlear blood flow with more superficial circulation. Moreover, unlike cochlear imaging by intravital microscopy (31
), FME does not require extensive resection of surrounding anatomy such as middle ear structures. One current limitation to our methodologies concerns the mechanical rigidity of the endoscope probe. The probe cannot bend within the geometric constraints imposed by the cochlear turns. The use of angled endoscope probes might partly alleviate this issue by providing an assortment of viewing angles but would still not allow the probe to be inserted through multiple cochlear turns.
We hope that in the future FME might be used while maintaining inner ear function. A significant limitation is the need for a cochleostomy to introduce the endoscope probe. In the current study, we made no effort to preserve hearing in our experimental animals or to quantify the anatomic and physiologic sequelae of our intervention. We seek to refine the techniques presented here to minimize the cochleostomy diameter, to avoid direct contact of the probe with the basilar membrane or osseous spiral lamina, and then to assess the affect of FME on hearing. Many delicate features of the cochlea, such as the cochlea’s sharp frequency tuning and high sensitivity, quickly disappear when the inner ear is opened (14
). Nevertheless, investigators have been able to access the guinea pig cochlea at both basal and apical ends without substantial loss of hearing (52
). Otologic surgeons routinely open the inner ear while preserving hearing during stapedectomy (58
), and it has also been demonstrated that cochlear implantation can be performed while maintaining useful acoustic function of the implanted cochlea (61
). Such findings make us think that it may be possible to perform cochlear FME while preserving hearing function.